1. Field of the Invention
The present invention relates to a toroidal-type continuously variable transmission for use as a transmission in a vehicle such as a car or the like.
2. Description of the Related Art
Conventionally, as a transmission for a vehicle such as a car or the like, a transmission which has a plurality of gears and changes engagement among these gears so as to transmit torque from an input shaft to an output shaft has been mainstream. In such a conventional gear-type stepwise variable transmission, however, the torque changes stepwise or intermittently at the time of change gear. As a result, there has been such a problem that loss is produced in power transmission or vibration is generated at the time of change gear.
Therefore, recently, a continuously variable transmission in which there is no stepwise or intermittent change in torque at the time of change gear has been put into practical use. In this continuously variable transmission, there is an excellent point that no vibration is generated at the time of change gear or the like and loss in power transmission is lower than that in the above-mentioned gear-type stepwise variable transmission. In addition, the continuously variable transmission is superior in the cost of fuel when it is used in a vehicle. Accordingly, a belt-type continuously variable transmission which is an example of the above-mentioned continuously variable transmission has been put into practical use for a part of passenger cars or the like.
In addition, as another example of the continuously variable transmission, there has been proposed a toroidal-type continuously variable transmission constituted by an input disk which rotates while interlocking with an input shaft, an output disk which rotates while interlocking with an output shaft, and a power roller bearing which is in rotary contact with these input and output disks.
This toroidal-type continuously variable transmission can transmit high torque in comparison with the above-mentioned belt-type continuously variable transmission. Therefore, the toroidal-type continuously variable transmission is regarded as effective particularly as a continuously variable transmission for a medium-sized or large-sized vehicle.
However, since such a toroidal-type continuously variable transmission is required to transmit higher torque, the input or output disk or the power roller bearing suffers very large repeated bending stress or very large repeated shearing stress in comparison with mechanical parts such as general gears, bearings and soon to which usual repeated stress is applied.
In the case of a general bearing, the number of balls is about 10 and these balls share a load equally. Shoulder portions of inner and outer peripheral portions of the bearing are supported by a shaft and a housing. Therefore, bending stress is extremely small.
On the other hand, in the case of the toroidal-type continuously variable transmission, each disk comes into contact with the power roller at two places so that two- or three-point press is produced. In addition, the disk is supported on an inner peripheral portion and an end surface near the inner peripheral portion. Therefore, particularly when the contact point comes to the outer periphery side of the disk, the value of the bending stress becomes extremely high in comparison with that in the general bearing.
FIGS. 4A to 4C show a disk 50 and a power roller bearing 51 of a toroidal-type continuously variable transmission, and a general roller bearing 52, where hatched portions designate stress suffering portions.
According to FEM analysis upon some specified disks of toroidal-type continuously variable transmissions, there appear portions where the pulling stress is in a range of from 70 kgf/mm2 to 80 kgf/mm2 in response to the input of 370 Nxc2x7m. Therefore, the allowable stress varies much in accordance with the values of residual compressive stress, hardness and surface roughness in such a portion.
A general bearing is usually used with contact surface pressure Pmax in a range of from 2 GPa to 3 GPa, while a toroidal-type continuously variable transmission is usually used with contact surface pressure of a traction portion in a range of from 2.5 GPa to 3.5 GPa. Particularly at the time of maximum speed reduction, the contact surface pressure of an input disk reaches 3.9 GPa in some specified toroidal-type continuously variable transmissions.
In a single-row ball bearing in which disks have substantially the same outer diameter, for example, in a 6315 ball bearing (outer diameter: 160 mm), a calculated contact ellipse has a major diameter of 6.86 mm and a minor diameter of 0.89 mm when the contact surface pressure Pmax reaches a load of 3.9 GPa which is close to a limit.
On the other hand, in a traction portion of a toroidal-type continuously variable transmission with some specifications, a contact ellipse has a major diameter of 5.15 mm and a minor diameter of 1.33 mm when the reduction ratio is 2.0, the input torque is 370 N.m, and the contact surface pressure Pmax on the input disk side is 3.9 GPa. When the contact surface pressure Pmax on the output disk side is 2.93 GPa under the same conditions, a contact ellipse has a major diameter of 4.77 mm and a minor diameter of 1.91 mm.
Thus, in the case of the toroidal-type continuously variable transmission, the minor diameter of the contact ellipse becomes larger than that in the general bearing. Therefore, there is required a surface hardening layer (Zo) which is deep enough to prevent the maximum shearing stress portion from causing fatigue failure.
Since the toroidal-type continuously variable transmission transmits power through the shearing stress of traction oil, high heat is generated in a contact portion thereof. The contact point temperature is estimated to exceed 200xc2x0 C., and the oil film thickness has a value of comma several microns on calculation.
Since spin is inevitably present in the toroidal-type continuously variable transmission structurally, heat is generated. Although spin is produced even in an angular bearing or a thrust ball bearing so that heat is generated, the toroidal-type continuously variable transmission differs much from the general bearing in the point where power of about 50 kw is transmitted per small contact ellipse of only about 5 mmxc3x971.5 mm. Therefore, the proof stress in a contact portion has to be higher than that in the general bearing.
Thus, it is necessary to transmit very high torque in the toroidal-type continuously variable transmission. Therefore, the input and output disks and the inner and outer races of the power roller bearing which are constituent members of the toroidal-type continuously variable transmission, should be manufactured with material having higher durability than that of conventional general mechanical parts.
It is therefore an object of the present invention to provide a toroidal-type continuously variable transmission which can satisfy such a request.
According to the invention, there is provided a toroidal-type continuously variable transmission, comprising: an input disk interlocking with an input shaft; an output disk interlocking with an output shaft; and a power roller bearing including an inner race, an outer race and a plurality of rolling elements, the inner race engaging with the input disk and the output disk to thereby transmit power of the input shaft to the output shaft. At least one of components of the toroidal-type continuously variable transmission including the inner and outer races of the power roller bearing, the input disk and the output disk is composed of an alloy steel material containing C: 0.15 to 0.5 wt %, Si: 0.1 to 1.5 wt %, Mn: 0.1 to 1.5 wt %, Cr: 0.5 to 3.0 wt %, and at least one kind selected from Mo: 0.1 to 3.0 wt % and V: 0.1 to 3.0 wt %, a content of O in the alloy steel material being set to be not more than 9 ppm. The surface of the at least one of components after carbonitriding, hardening and tempering treatments is set to contain C: 0.8 to 1.2 wt % and N: 0.05 to 0.20 wt %. The at least one of components has surface hardness of Hv 720 or more. And, Mo.V carbide/carbonitride, the average particle size of which is in a range of from 50 nm to 500 nm is dispersed and precipitated on a race surface or in a range of from the race surface to a deep position where a maximum shearing stress is generated.